94 FRESH-WATER BIOLOGY 



The addition of glucose or nitrate, for example, to ordinary nutrient 

 broth will enable certain species of bacteria to grow under condi- 

 tions otherwise unfavorable. The relation between anaerobic life 

 and food supply is an intimate one. The anaerobes, in a word, are 

 those organisms able to obtain their needed energy from the simple 

 splitting of organic compounds without oxidation. If a microorgan- 

 ism is so specialized to an anaerobic mode of life that the presence 

 of oxygen, except in minute quantities, interferes with its habitual 

 method of attacking food substances, it is an obligatory anaerobe. 

 In a modified form, therefore, Pasteur's conception of fermentation 

 as "life without air" is not very far from the modern view. 



Those decompositions of organic substances that are usually 

 termed putrefactions and are characterized by the evolution of 

 malodorous gases such as hydrogen sulphide and the production of 

 substances like skatol, indol, mercaptan, etc., are due to the agency 

 of anaerobic bacteria. In fact, researches indicate that the putre- 

 factive decomposition of native proteins is wholly the work of the 

 obligatory anaerobes. As is well known, the ooze at the bottom of 

 ponds and streams is peculiarly the home of such anaerobic decom- 

 positions. 



Bacteria are everywhere present in natural bodies of water. 

 They are more abundant as a rule in surface waters than in ground 

 waters. Deep well waters and spring waters in certain regions 

 often contain very few bacteria, perhaps only five to ten per cubic 

 centimeter, while the water of lakes and ponds usually contains 

 several hundred, and ordinary river water contains numbers that at 

 times rise into the thousands and tens of thousands. As a general 

 rule, sewage-polluted waters contain more bacteria than pure waters. 

 An excessively polluted stream, such as the Chicago River once 

 was, may hold as many as several million bacteria per cubic centi- 

 meter. 



The number of bacteria in a river water varies greatly at differ- 

 ent seasons of the year, being generally larger in the colder months 

 than in summer. Probably this is due in part to the winter in- 

 crease in current caused by rains and melting snows which prevents 

 sedimentation; in part to the heavy rains of winter which wash into 

 a stream numberless germs from cultivated lands, and partly also 



